1. Field of Invention
The invention relates to a catalyst degradation determining method for determining whether a catalyst disposed in an exhaust passage of an internal combustion engine has degraded.
2. Description of Related Art
A known three-way catalyst (in this specification, sometimes referred to simply as “catalyst”) for controlling exhaust gas from an internal combustion engine is disposed in an exhaust passage of the internal combustion engine. The three-way catalyst performs the function of oxidizing unburned components (HC, CO) (oxidizing function), and the function of reducing nitrogen oxides (NOx) (reducing function), and is able to substantially remove harmful components, including unburned components and nitrogen oxides mentioned above, due to the oxidizing and reducing functions.
The efficiency of removal of harmful components by the oxidizing and reducing functions of the catalyst is known to rise as the air-fuel ratio of the internal combustion engine approaches a stoichiometric air-fuel ratio. If the air-fuel ratio of the internal combustion engine is kept within a predetermined range (hereinafter, referred to as “window range”) that contains the stoichiometric air-fuel ratio, the removal efficiency can be kept at or above a predetermined high value.
The three-way catalyst further has the function of oxidizing unburned components, such as HC, CO, etc., with oxygen released from the catalyst if the exhaust gas coming into the three-way catalyst has a fuel rich-side air-fuel ratio (oxygen releasing function), and the function of storing oxygen released from nitrogen oxides (NOx) if the incoming exhaust gas has a fuel lean-side air-fuel ratio (oxygen storing function). Due to the oxygen releasing and storing functions, the three-way catalyst is able to substantially remove harmful components including unburned components and nitrogen oxides mentioned above. Therefore, the emissions control capability of the three-way catalyst increases with increases in the maximum value of the amount of oxygen storable in the three-way catalyst. Hereinafter, the amount of oxygen storable in the three-way catalyst will be referred to as “oxygen storage amount”, and the maximum value thereof will be referred to as “maximum oxygen storage amount”.
The catalyst degrades due to heat given to the catalyst or the poisoning by lead, sulfur and the like contained in fuel. As the degradation of the catalyst progresses, the maximum oxygen storage amount decreases. Therefore, if the maximum oxygen storage amount of the catalyst is estimated, it becomes possible to determine whether the catalyst has degraded on the basis of the estimated maximum oxygen storage amount. It is to be understood that “storage” used herein means retention of a substance (solid, liquid, gas molecules) in the form of at least one of adsorption, adhesion, absorption, trapping, occlusion, and others.
A catalyst degradation degree detecting apparatus disclosed in Japanese Patent Application Laid-Open Publication No. 5-133264 is provided on the basis of the aforementioned finding. That is, the air-fuel ratio of the internal combustion engine is forcibly changed from a fuel-lean air-fuel ratio to a predetermined fuel-rich air-fuel ratio (or from a fuel-rich air-fuel ratio to a predetermined fuel-lean air-fuel ratio). On the basis of a corresponding change in the output of the air-fuel ratio sensor disposed downstream of the catalyst, the maximum oxygen storage amount of the catalyst is estimated. A degree of degradation of the catalyst is detected on the basis of the estimated maximum oxygen storage amount.
More specifically, the disclosed apparatus first sets the oxygen storage amount at the maximum oxygen storage amount by controlling the upstream-of-catalyst air-fuel ratio to a lean-side air-fuel ratio, and then controls the air-fuel ratio of the catalyst to a predetermined rich-side air-fuel ratio. By multiplying the time elapsing until the oxygen storage amount of the catalyst reaches “0” and the output-of the air-fuel ratio sensor disposed downstream of the catalyst changes to a rich side (hereinafter, the time point of the output of the air-fuel ratio changing to the rich side will be referred to as “the time of air-fuel ratio sensor output rich-side switch”) by the amount of oxygen released (consumed) in the catalyst per unit time, the apparatus estimates a maximum oxygen storage amount (hereinafter, this estimation method is referred to as “rich air-fuel ratio-based estimation method”). In another method, the upstream-of-catalyst air-fuel ratio is controlled to a rich air-fuel ratio so as to set the oxygen storage amount of the catalyst at “0”. After that, the upstream-of-catalyst air-fuel ratio is controlled to a predetermined lean-side air-fuel ratio. By multiplying the time elapsing until the oxygen storage amount of the catalyst reaches or exceeds the maximum oxygen storage amount and the output of the air-fuel ratio sensor disposed downstream of the catalyst changes to the lean side (hereinafter, the time point of the output of the air-fuel ratio changing to the lean side will be referred to as “the time of air-fuel ratio sensor output lean-side switch”) by the amount of oxygen inflow to the catalyst per unit time, a maximum oxygen storage amount is estimated (hereinafter, this estimation method is referred to as “lean air-fuel ratio-based estimation method”). That is, this apparatus determines a maximum oxygen storage amount by using at least a change in the output of the air-fuel ratio sensor disposed downstream of the catalyst, and the predetermined lean air-fuel ratio or the predetermined rich air-fuel ratio in, for example, a case where there is a need to estimate the maximum oxygen storage amount again, or the like.
If the maximum oxygen storage amount of the catalyst is estimated by the rich air-fuel ratio-based estimation method, supply of a mixture of the predetermined rich air-fuel ratio is continued until the aforementioned air-fuel ratio sensor output rich-side switch occurs. In this case, as the maximum oxygen storage amount of the catalyst is estimated at the time of the air-fuel ratio sensor output rich-side switch, it is no longer necessary to keep the upstream-of-catalyst air-fuel ratio at an air-fuel ratio that is rich of the stoichiometric air-fuel ratio. If the upstream-of-catalyst air-fuel ratio is kept rich of the stoichiometric air-fuel ratio immediately after the air-fuel ratio sensor output rich-side switch, unburned components, such as CO, HC and the like, are readily discharged since the oxygen storage amount of the catalyst is “0” and the oxygen releasing function of the catalyst is not effective. Therefore, after the air-fuel ratio sensor output rich-side switch occurs, it is preferable to set the upstream-of-catalyst air-fuel ratio at the stoichiometric air-fuel ratio or set the upstream-of-catalyst air-fuel ratio at an air-fuel ratio that is lean of the stoichiometric air-fuel ratio.
However, at the time of air-fuel ratio sensor output rich-side switch, a space defined by the catalyst and the exhaust passage from the exhaust port of the internal combustion engine to the air-fuel ratio sensor disposed downstream of the catalyst is filled with a gas having a predetermined rich air-fuel ratio. If in this case, the predetermined rich air-fuel ratio is considerably below a lower limit value of the aforementioned window range, unburned components contained in the gas filling the aforementioned exhaust passage and the like, that is, CO, HC, etc., are great in amount. Furthermore, since the oxygen releasing function of the catalyst is not effective and the efficiency of removal of unburned CO, HC by the oxidizing function of the catalyst is low, large amounts of unburned CO and HC are emitted into the atmosphere immediately after the air-fuel ratio sensor output rich-side switch occurs although the upstream-of-catalyst air-fuel ratio immediately following the air-fuel ratio sensor output rich-side switch is set at the stoichiometric air-fuel ratio or an air-fuel ratio lean of the stoichiometric air-fuel ratio.
Therefore, if the maximum oxygen storage amount of the catalyst is estimated by the rich air-fuel ratio-based estimation method, it is preferable to set the aforementioned rich air-fuel ratio at an air-fuel ratio that is rich of the stoichiometric air-fuel ratio and that is equal to or higher than the lower limit value of the window range in order to lessen the amount of unburned CO, HC emitted immediately after the air-fuel ratio sensor output rich-side switch occurs.
Similarly, if the maximum oxygen storage amount of the catalyst is estimated by the lean air-fuel ratio-based estimation method, the supply of a mixture having the aforementioned predetermined lean air-fuel ratio is continued until the air-fuel ratio sensor output lean-side switch occurs. In this case, as the maximum oxygen storage amount of the catalyst is estimated at the time of the air-fuel ratio sensor output lean-side switch, it is no longer necessary to keep the upstream-of-catalyst air-fuel ratio at an air-fuel ratio that is lean of the stoichiometric air-fuel ratio. If the upstream-of-catalyst air-fuel ratio is kept lean of the stoichiometric air-fuel ratio immediately after the air-fuel ratio sensor output lean-side switch occurs, nitrogen oxides NOx are likely to be emitted since the oxygen storage amount of the catalyst has reached the maximum oxygen storage amount and the oxygen storing function of the catalyst is not effective. Therefore, after the air-fuel ratio sensor output lean-side switch occurs, it is preferable to set the upstream-of-catalyst air-fuel ratio at the stoichiometric air-fuel ratio, or set the upstream-of-catalyst air-fuel ratio at an air-fuel ratio that is rich of the stoichiometric air-fuel ratio in, for example, a case where there is a need to estimate the maximum oxygen storage amount again, or the like.
However, at the time of air-fuel ratio sensor output lean-side switch, the space defined by the catalyst and the exhaust passage from the exhaust port of the internal combustion engine to the air-fuel ratio sensor disposed downstream of the catalyst is filled with a gas having a predetermined lean air-fuel ratio. If in this case, the predetermined lean air-fuel ratio is considerably above an upper limit value of the aforementioned window range, nitrogen oxides NOx contained in the filling gas are great in amount. Furthermore, since the oxygen storing function of the catalyst is not effective and the efficiency of removal of nitrogen oxides NOx by the reducing function of the catalyst is low, a large amount of nitrogen oxides NOx is emitted into the atmosphere immediately after the air-fuel ratio sensor output lean-side switch occurs although the upstream-of-catalyst air-fuel ratio immediately following the air-fuel ratio sensor output lean-side switch is set at the stoichiometric air-fuel ratio or an air-fuel ratio that is rich of the stoichiometric air-fuel ratio.
Therefore, if the maximum oxygen storage amount of the catalyst is estimated by the lean air-fuel ratio-based estimation method, it is preferable to set the aforementioned lean air-fuel ratio at an air-fuel ratio that is lean of the stoichiometric air-fuel ratio and that is equal to or lower than the higher limit value of the window range in order to lessen the amount of nitrogen oxides NOx emitted immediately after the air-fuel ratio sensor output lean-side switch occurs.
It is known that as the degradation of the catalyst progresses, the maximum oxygen storage amount decreases, and moreover, the efficiency of removal of harmful components by the oxidizing and reducing functions of the catalyst with respect to a fixed air-fuel ratio (oxidizing/reducing capability) decreases, and the window range of the catalyst narrows. It is also known that the efficiency of removal of harmful components by the oxidizing and reducing functions of the catalyst with respect to a fixed air-fuel ratio and the window range of the catalyst also change depending on the temperature of the catalyst.
Therefore, if the maximum oxygen storage amount of the catalyst is estimated by the rich air-fuel ratio-based estimation method or the lean air-fuel ratio-based estimation method while using the assumption that the window range is constant, and that the predetermined rich air-fuel ratio is a constant air-fuel ratio which is within the window range and near the lower limit value of the window range, or that the predetermined lean air-fuel ratio is a constant air-fuel ratio which is within the window range and near the upper limit value of the window range, the following problem may occur. That is, as the degradation of the catalyst progresses, the predetermined rich air-fuel ratio or the predetermined lean air-fuel ratio becomes an air-fuel ratio that is outside the window range, and it becomes impossible to lessen the amount of harmful components emitted immediately after the switch of the output of the downstream-of-catalyst air-fuel ratio sensor occurs.
The amount of harmful components emitted immediately after the switch of the output of the downstream-of-catalyst air-fuel ratio sensor occurs can be lessened if the predetermined rich air-fuel ratio or the predetermined lean air-fuel ratio is set beforehand at an air-fuel ratio that is near the stoichiometric air-fuel ratio. However, this measure creates a problem of a prolonged time elapsing from the beginning of control of the upstream-of-catalyst air-fuel ratio to the predetermined rich air-fuel ratio or lean air-fuel ratio to the switch of the output of the downstream-of-catalyst air-fuel ratio sensor (i.e., a prolonged period results for calculation of the maximum oxygen storage amount of the catalyst).